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lithium titanate composite as anode material for lithium ion batteries
Electrochimica Acta 155 (2015) 125–131
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Evaluation of the electrochemical characteristics of silicon/lithium titanate composite as anode material for lithium ion batteries Jing Shi a , Yunhui Liang a , Linlin Li a , Yi Peng a , Huabin Yang a,b, * a Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China b Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 December 2014 Received in revised form 26 December 2014 Accepted 27 December 2014 Available online 30 December 2014
Silicon/lithium titanate (Si/Li2TiO3) nanocomposite is successfully prepared through the combination of a sol–gel approach with a high-temperature treatment as well as a high energy ball milling process. The structure and morphology of the composite are characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) analysis reveals Si particles are coated by the uniform disordered Li2TiO3 layer with a thickness of about 5 nm. The investigation in cycling performances demonstrates that Si/Li2TiO3 exhibits the improved cycling stability, with specific capacity of 471.0 mA h g1 after 50 cycles and the capacity retention is 31.5%, much higher than pure Si. Compared with pure Si, Si/Li2TiO3 shows better rate-capability, a reversible capacity of 315.2 mA h g1 at 0.8 A g1 is maintained. The higher ionic conductivity of Li2TiO3 is responsible for the improved rate performance. In addition, the results derived from XRD, the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicate that lithium ions could react reversibly with Si, and electrochemically less active Li2TiO3 turns into the Li–Ti–O ternary phase, which acts as a buffer matrix in the Si/Li2TiO3 composite, thus improving the reversibility of electrode. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Silicon/lithium titanate composite Lithium titanate matrix Electrochemical performance Anode Lithium ion battery
1. Introduction In the rapid development of portable electronic devices, lithiumion batteries (LIBs) are strongly demanded in consideration of their high energy density, long cycle life and fast charge/discharge rates. Graphite, currently used as lithium-ion battery anode, is increasingly approaching its theoretical capacity limit of 372 mA h g1, however, it is far from meeting the demands of future electronic equipment [1]. Therefore, a variety of anode materials with improved storage capacity have been intensively explored for lithium-ion batteries in the last decade. Among these, silicon has aroused extensive attention for its numerous appealing features: it has the highest specific capacity of 4200 mA h g1 [2] and is inexpensive, abundant, furthermore, the lithiation/delithiation potential of Si is slightly higher than that of lithium, resulting in the electrode with safe feature. However, there are two major problems of Si anode material: the low intrinsic electronic conductivity and huge volume expansion during Li insertion/extraction processes, resulting in cracking and crumbling of the electrode, which makes it retain poor cycling performance [3–5]. Tremendous efforts have been made to
* Corresponding author. Tel.: +86 22 23508405; fax: +86 22 23502604. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.electacta.2014.12.153 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
overcome these drawbacks, including: 1) using nanotechnologies to synthesize different Si nano/micro-structures, such as nanoparticles [6,7], nanotubes [8,9], thin films [10] and mesoporous structures [11,12]; 2) coating with carbon (C) to form Si/C composites [13,14]; 3) combining with inactive/active materials [15,16], such as metal alloys [17,18] and metal oxides [19,20]. Among other alternative anode materials, Ti-based compounds are particularly attractive for their excellent structural stability during cycling and higher flat electrode reaction voltage. Lithium titanate has emerged as a good candidate as anode material for lithium-ion batteries with higher and flatter Li insertion reaction voltage, preventing the formation of Li dendrites and the decomposition of electrolyte [21–26]. Considering the merits emerged above, anodes made of silicon/lithium titanate composites can combine the advantageous properties of silicon (high lithium-storage capacity) and lithium titanate (excellent structural stability) to improve the overall electrochemical performance of the anode for lithium-ion batteries. There have also been some researches on the composites consisting of silicon and lithium titanate in the previous reports. For example, Si/LiTi2O4 composite film was synthesized by a sol–gel method in combination with a following heat-treatment process using nano-Si powder as the raw material. The composite exhibited excellent cycle performance (1100 mA h g1 remained after 50 cycles) but a low initial coulombic efficiency
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(only about 74.8%). It was reported that the improvement of electrochemical performance was mainly induced by the porous structure of the Si/LiTi2O4 nanocomposite film, which could offer convenient channels and provided a buffer interspace to alleviate the volume expansion of Si during the cycle process [27]. Yingbin Lin et al. [28] synthesized composite a-Si film/Li4Ti5O12 via vacuum thermal evaporation technique, which shows better cycling performance than that of Li4Ti5O12 in the voltage range of 1.0–3.0 V. However, the vacuum thermal evaporation technique is difficult to achieve from the point of view of large-scale commercial applications. Li2TiO3 owns a cubic structure and can react with Li atoms to form zero-strain insertion materials. Moreover, Li2TiO3 has a threedimensional path for Li+-ion diffusion, inwhich Li+-ion migration can take place in (0 0 3) plane and along c direction [29]. The structural stability and its better ionic conductivity make it act as ideal buffer to the Si powder. Besides, the introduction of carbon can further improve the electrical conductivity of the Si/Li2TiO3 composite. In this paper, the synthesis of Si/Li2TiO3 composite is reported by dispersing micro-Si powder in Li2TiO3 matrix by a facile sol–gel method followed by a high-temperature solid-phase calcination process and high-energy ball-milling technique. According to our knowledge, there are few reports considering the effect of Li2TiO3 on the electrochemical performance of pure Si. In this work, the introduction of Li2TiO3 endows the Si/Li2TiO3 electrode with the excellent cycling stability and better rate-capability compared with pure Si. The improved electrochemical performance of the Si/Li2TiO3 electrode might be attributed to the buffer action of the Li2TiO3 matrix and the higher ionic conductivity of Li2TiO3.
ingredients dissolved in N-methyl pyrrolidinone (NMP) onto copper foil and then dried at 80 C under vacuum for 24 h. The working electrodes were assembled in 2032 coin cells using Celgard 2400 as the separator and lithium foil as the counter electrodes. 1 mol L1 LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume ratio) was employed as the electrolyte. The coin-type cells were assembled in an argon-filled glove box. The cyclic voltammetry (CV) measurement was performed on a Solartron 1287 electrochemical workstation between 0 and 3.0 V at a scan rate of 0.1 mV s1, and the test temperature was kept at 25 C. Electrochemical impedance spectroscopic (EIS) measurement was carried out by utilizing an AC voltage of 5 mV amplitude of in the frequency range of 10 kHz to 100 MHz using a Solartron 1250 impedance analyzer, and the test temperature was kept at 25 C. The galvanostatic charge/discharge tests were conducted on a LAND CT2001A battery test system at 25 C with the cut-off voltage of 0.02 and 3.0 V (vs. Li/Li+) at a specific current density of 80 mA g1. Especially, the rate test of pure Si and Si/Li2TiO3 composite at different current densities was in the voltage of 0.01 and 3.0 V (vs. Li/Li+). 3. Results and discussion 3.1. Structure characterization Fig. 1 shows the TG-DTA curves of the glucose and the Si/Li2TiO3 precursor. It can be clearly seen that four main endothermic peaks are observed at around 74, 157, 220 and 313 C in the DTA curve in
2. Experimental 2.1. Preparation of Si/Li2TiO3 composite For a typical synthesis of Si/Li2TiO3 composite, lithium acetate and tetrabutyl titanate with a molar ratio of 4:5 (Li:Ti) were dissolved in an ethanol solution. Then an appropriate amount of triethanolamine was added into the above solution, followed by an ethanol solution containing Si powder with a particle size of 1–20 mm and the molar ratio of Si to Ti was 2:1. Subsequently, an aqueous solution of glucose (C6H12O6H2O) with water to glucose molar ratio of 9:1, was added dropwise into the mixture under stirring. A suspension was achieved, then dried at 80 C for 10 h to form a dry gel precursor. The resulting precursor was pressed into round slice under pressure of 10 MPa and then heated at 700 C in argon atmosphere for 24 h. Finally, the obtained black powder was ball-milled at 400 rpm for 6 h in a 100 mL ceramic vial filled with ceramic balls and argon gas. The weight ratio of the ceramic balls to the material was maintained at 20:1. 2.2. Structural Characterization The phase of the as-prepared powders were characterized by X-ray diffraction (XRD) using a Rigaku D/max-2500 X-ray diffractometer equipped with Cu-Ka radiation. The morphologies and microstructures of the synthesized materials were characterized by a transmission electronic microscopy (TEM, FEI Tecnai20) and a scanning electron microscopy (SEM, Philips XL-30). The elementary composition was primarily determined by energy dispersive spectrum (EDS). The pyrolysis performance of the samples was studied by a thermal gravimetric analyzer (TG, SDTQ600). 2.3. Electrochemical characterization Electrodes containing Si, or Si/Li2TiO3 active materials, polyvinylidene fluoride (PVDF) binder and carbon black at a weight ratio of 8:1:1, were prepared by coating the slurry of the electrode
Fig. 1. TG-DTA curves of glucose (a) and the Si/Li2TiO3 precursor (b).
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Fig. 1(a), corresponding to the thermal behavior of glucose, which are resulted from departure of adsorbed water, fusion of the glucose and two periods of thermal decomposition of the glucose, respectively [30]. Therefore, it can be inferred that the new endothermic peak located at 497.6 C in Fig. 1(b) should be ascribed to the formation of Li–Ti–O ternary phase [31] while the other four major peaks below 350 C are relative to the thermal behavior of the glucose. The typical XRD pattern of Si/Li2TiO3 composite is displayed in Fig. 2. The strong and sharp diffraction peaks are indexed to Si and the major broaden ones can be ascribed to Li2TiO3, indicating the existence of well crystallized Si and low crystallinity Li2TiO3. However, no any peak can be indexed to the Li4Ti5O12 phase, which may be attributed to the mass loss of the rude materials during the preparation process as stated in the Experimental. As a result, the resultant Li2TiO3 doesn't match with the molar ratio of Li:Ti = 4:5. Besides, there is one discernable phase of SiO2 in the composite, which is attributed to the partial oxidation of Si. Fig. 3(a), (b) show SEM images of the Si/Li2TiO3 particles with different magnification. Many near-spherical particles are observed and this material has the typical morphology of materials prepared by sol–gel method [32] from the low-magnified SEM image (Fig. 3(a)). In the high-magnified SEM image (Fig. 3(b)), near-spherical particles can be seen with the size of 20–450 nm, including a small amount of large aggregated particles formed by the aggregation of tiny particles. Fig. 3(f) shows the energy dispersive spectrometer (EDS) pattern of the composite. It can be seen that the elements Cu, S and Si, Ti, and O, can be detected, which are originated from the Cu substrate, the contaminants of Cu substrate, and Si/Li2TiO3 particles, respectively. The EDS analysis reveals that the atom ratio of O/Ti in the Si/Li2TiO3 composite is about 3.7, which further confirms the existence of Li2TiO3 instead of Li4Ti5O12, and the molar ratio of Si to Li2TiO3 is about 3.3 (Table 1). The slight excess of O can be ascribed to the existence of O atom of SiO2. In order to further identify the Si and Li2TiO3 layer, the TEM and HR-TEM images were employed. Si presents the morphology of particles with average particle size of more than 100 nm and the uniform disordered Li2TiO3 layer with a thickness of about 5 nm can be observed in the Si/Li2TiO3 composite (Fig. 3(d) and (e)), which has been mentioned to act as a substrate for Si. Moreover, In Fig. 3(e), orderly lattice fringes are observed and the measured lattice spacing is 0.31 nm, which matches well with the (111) diffraction plane of Si. It is reasonable to state that the ordered structure is crystalline silicon. To analyze any changes in the microstructure or morphology of the composite during cycling, comparison of morphology of electrode materials before cycling and after cycling for 30 cycles are clearly shown in SEM images (Fig. 3(b) and (c)), which exhibit very little change in surface
Fig. 2. XRD pattern of Si/Li2TiO3 composite.
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morphology. Despite of the volume change, the Si/Li2TiO3 composite remains intact without breaking into smaller particles. The SEM images indicate that the overall particles could become more compact after cycling. As a result, it is particularly beneficial for maintaining the stability of the electrode materials. This is part of the reason why the good electrochemical performance can be expected. 3.2. Electrochemical performance Fig. 4displays the charge/discharge profiles of pure Si and Si/Li2TiO3 composite to compare the charge and discharge details of Si before and after the introduction of Li2TiO3. It can be seen that the charge and discharge platforms are a little different. In the sample of Si/Li2TiO3 composite, the lithium is inserted and extracted at the similar potential of Si, implying the Si plays a key role in providing most of the capacity. However, it is observed that most of the capacity loss comes from Si. While in contrast to the Si electrode, the Si/Li2TiO3 composite electrode reveals significantly enhanced cycle stability. The improved electrochemical performance can be revealed by comparing the cycling performances of pure Si and Si/Li2TiO3 composite. Fig. 4(c) compares the specific capacities of pure Si, Si/Li2TiO3 composite between 0.02 and 3.0 V up to 50 cycles. As for the pure Si, the capacity decays to 17.0 mA h g1 after 50 cycles, with capacity retention of 0.55%. It is obvious to observe a sharp drop in capacity in the 11th cycle, which can be attributed to the cracking and pulverization of Si electrode [33]. After introduced by Li2TiO3, its capacity retention is changed to 31.5%, and the retained capacity reaches 471.0 mA h g1 after 50 cycles, suggesting that the Si/Li2TiO3 electrode exhibits excellent cycle stability. Besides, it is obvious that the introduction of the Li2TiO3 significantly improves the capacity of pure Si. The Si/Li2TiO3 composite displays an initial charge capacity of 1199.3 mA h g1 and discharge capacity of 1497.9 mA h g1, an initial coulombic efficiency of 80.1%, which is much higher than that of pure Si. One possibility of the improved performance may be assigned to the buffer action of the Li2TiO3 matrix. Li2TiO3 as a zero-strain insertion material has no structural change during lithium insertion/ extraction and can effectively withstand drastic volume expansion of Si during the cycling. Besides, the Li–Ti–O ternary phase can also partly compensate the capacity loss through providing approximate 100 mA h g1 reversible lithium storage capacity [25,26,34]. In the subsequent cycles, the coulombic efficiency remains at about 97%. However, as shown in Fig. 4(d), the capacity is kept at nearly 300 mA h g1 after 100 cycles, this instability may be accounted to the following three reasons: 1) The inappropriate Li2TiO3 in the composite may not completely act as a buffer layer against the volume change of Si during repeated alloying and de-alloying; 2) The volume effect of Si can't be ignored due to the larger size of Si nanoparticles. Moreover, judging from Fig. 4(a) and (b), most of the capacity loss is caused by Si; 3) In this paper, the thermal decomposition of LiPF6 based electrolyte without other additives and the reactions of the electrolyte with the surface of the electrode materials limit the thermal stability of LIBs [35]. Fig. 5 exhibits the rate performances of pure Si and Si/Li2TiO3 composite at different current densities in the voltage range of 0.01–3.0 V. Compared with pure Si, the Si/Li2TiO3 composites shows better rate-capability. In particular, it retains a high capacity of 315.2 mA h g1 at 0.8 A g1, while that of Si is only 4 mA h g1, suggesting the much improved rate capability. Li-conductive Li2TiO3 and the 3D path for Li+-ion diffusion [36] are proposed as the reasons for a better rate capacity of Si/Li2TiO3 composite. In order to clearly understand the electrochemical properties of the lithiation and delithiation reaction, the CV, XRD and EIS tests were performed on the Si/Li2TiO3 electrode. Fig. 6 shows the CV curves of the Si/Li2TiO3 electrode at a scanning rate of 0.02 mV s1 in the potential range from 0 to 3.0 V (vs. Li+/Li). During the initial
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Fig. 3. SEM micrograph of the Li2TiO3 composite before cycling (magnification: 10000) (a), before cycling (magnification: 200000) (b) and after 30 cycles (c); TEM (d) HR-TEM (e) images of Si/Li2TiO3 particles; EDS phases (f) of Si/Li2TiO3 particles. Table 1 The results of element analysis in the composite. Element
Weight percent/%
Atomic percent/%
Si Ti
23.7 12.2
18.7 5.6
cycle, four cathodic peaks located at 1.586, 0.706, 0.123 and 0.01 V and three anodic peaks located at 0.41, 0.553 and 1.80 V are observed. The cathodic peak located at 0.706 V only emerging during the initial cycle should be attributed to the formation of the SEI film on the surface of the active particles and the decomposition of electrolyte [37,38]. The two cathodic peaks located at 0.123 and 0.01 V and the two corresponding anodic peaks located at 0.41 and 0.553 V should be ascribed to the
Fig. 4. Charge-discharge profiles of pure Si (a), Si/Li2TiO3 composite (b) and cycling performances of pure Si, Si/Li2TiO3 composite after 50 cycles (c). Si/Li2TiO3 composite after 100 cycles (d).
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Fig. 5. Rate performances of pure Si and Si/Li2TiO3 composite in the voltage range of 0.01–3.0 V.
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that of pure Si, the XRD patterns of the Si/Li2TiO3 electrode material discharging and charging to different voltages during the initial cycle are evaluated in Fig. 7. No obvious phase change has been observed when discharging from 1.586 V to 0.123 V, indicating that there is no obvious reaction between the active material Si and Li (Fig. 7(a)). However, when discharging deeply to 0.01 V (Fig. 7(b)), the XRD pattern is clearly different from the above mentioned, because the diffraction peaks of LiaSib as the main component are obviously observed while those of Si nearly disappear. The result implies that the reaction of Si with Li is apparent and most of them have been involved in the reaction when discharging to 0.01 V. When it comes to charging shown in Fig. 7(c), with the increase of the delithiation depth, the diffraction peak intensity of Si is increasing while that of LiaSib is decreasing. It can be noted that the potential interval from
reversible lithiation/delithiation reaction of Li with Si [38–40]. The reaction equation is described as follows. LixSi $ xLi+ + xe + Si
(1)
The appearance of the cathodic peak located at 1.586 V and the anodic peak located at 1.80 V suggests the existence of another reversible lithiation/delithiation reaction. Previous studies suggested Li–Ti–O ternary phase can experience an reversible Li+ ion insertion and extraction reaction in this potential interval [23,41– 43] and the lithiation reactions of Li1+xTi2-xO4 (0 X 1/3) with a spinel structure and TiO2 with a rutile structure are described as follows. Our results are in agreement with those of the previous studies. Li+ + e + Li[Li1/3Ti5/3]O4 $ Li2[Li1/3Ti5/3]O4 E = 1.56/1.65 V [44]
(2)
xLi+ + xe + TiO2 (anatase, s) $ LixTiO2 E = 1.73/2.10 V [45]
(3)
Therefore, the two peaks can be identified as the reversible lithiation and delithiation reaction of Li with Li2TiO3. The smaller area of the peaks implies the lower content of the Li2TiO3 phase in the composite, which induces the inconspicuous discharge plateau at ca. 1.5 V. Moreover, with the increase of CV scanning cycles before 15th cycle, the current value of redox peaks is gradually increasing but the potential value keeps consistent, indicating the increasing speed of lithiation/delithiation reaction. Consequently, the Si/Li2TiO3 composite with good structural stability is a promising anode material. To gain further insight into the phase change of the composite during alloying and de-alloying and to further understand why the cycling performance of Si/Li2TiO3 composite is better than
Fig. 6. Cycle voltammetry of Si/Li2TiO3 composite electrode at a scan rate of 0.2 mV s1 in the potential range from 0.0 to 3.0 V (vs. Li+/Li).
Fig. 7. XRD patterns of Si/Li2TiO3 composite during the initial lithiation/delithiation process at different voltages (a) lithiation (b) before cycling and after discharging to 0.01 V during the initial cycle (c) delithiation.
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0.41 to 1.8 V is the main stage in which LiaSib releases lithium ion. Finally, when charging to 3.0 V, the phase of LiaSib nearly disappears and Si reappears. In comparison with the XRD patterns of the Si/Li2TiO3 electrode before and after the first cycle, it is obvious that the main phases of Si and Li2TiO3 keep unchanged but portion of crystalline Si turns into amorphous structure after the lithiation/delithiation reactions. The structural change of Si leads to part of the irreversible capacity loss during the initial cycle [46]. Besides, the appearance and disappearance of the LiaSib binary phase in discharge and charge processes reveal the existence of some reversible reactions which can generate Li–Si binary phase. Meanwhile, as the CV results above show, the reaction between Li+ and Li2TiO3 are found, nevertheless, there is no obvious change of the Li2TiO3 intensity in XRD patterns shown, indicating that Li–Ti–O ternary phase acts as a buffer matrix in the Si/Li2TiO3 composite, which consists with the results obtained from the CV curves. Therefore, the main reversible reaction during the cycling process can be presented as follows. LixSi $ xLi+ + xe + Si
(4)
Lix + 2TiO3 $ xLi+ + xe + Li2TiO3
(5)
To further clarify the origin of the superior lithium storage capacity and cycling performance, EIS analyses were conducted for
the Si/Li2TiO3 composite at different potentials where redox peaks exist in CV curves during the initial cycle. The Nyquist plots of the Si/Li2TiO3 composite electrode at various charge–discharge states in the 1st cycle are presented in Fig. 8. The corresponding equivalent circuit is demonstrated as an insert in Fig. 8(b), as shown, the semicircle that is associated with the interface electrochemical polarization impedance (Rp) and a double layer capacitance at the interface between the active material and the electrolyte, is represented by Rp and CPE (a constant phase element), separately, and Re is the resistance of the electrical contacts, separator and electrolyte [47]. It can be seen clearly that all the plots possess a depressed semicircle related with the interface electrochemical polarization impedance (Rp) in high frequency (HF) and an oblique line associated with Warburg impedance (W0) in low frequency. The semicircle diameter in HF corresponds to the Rp values, which means charge transfer resistance. The Rp value raises as the lithiation depth (Fig. 8(a)) increases and it actually increases by 22.9 V mg when discharging potential reduces from 1.586 to 0.706 V. According to the CV results, it is easier to infer that the formation of the SEI film reduces the electrochemical reaction activity. In addition, the Rp value increases by 17.3 and 20.9 V mg when the discharge voltage range from 0.706 to 0.123 V and further to 0.01 V, respectively. The increase of the Rp should be attributed to the formation of Li–Si binary phase in the lithiation reaction of Si to LiaSib,and this potential interval of the lithiation reaction exsits in the XRD results. Nevertheless, the Rp decreases (Fig. 8(b)) with the increase of delithiation depth. The Rp reduces by 20.3 and 38 V mg after charging from 0.41 to 0.553 V and further to 1.8 V, respectively. The reduction of Rp mainly originates from the reversion of the LiaSib. Therefore, the conclusion can be drawn that the Si/Li2TiO3 composite anode material possesses excellent reversibility during cycling, which is consistent with the XRD and CV results. 4. Conclusions In summary, a facile and effective approach for the synthesis of nano-sized Si/Li2TiO3 composite was demonstrated via the processes of sol–gel, calcination and ball-milling. In contrast to pure Si, the cycle stability and rate capability of Si/Li2TiO3 composite are much better. It shows a capacity retention of 31.5% and a coulombic efficiency of 80.1%, along with the retained capacity of 471.0 mA h g1 after 50 cycles, which are higher than pure Si. The improved cyclability of the Si/Li2TiO3 electrode is mainly due to the buffer action of the Li2TiO3 matrix, which also ensures the uniform dispersion of Si. Additionally, the Li–Ti–O ternary phase can partly compensate the capacity loss through providing approximate reversible lithium storage capacity. The higher ionic conductivity of Li2TiO3 might be the main reason for the improved rate performance. Furthermore, the lithiation and delithiation behaviors at different potentials have been proposed based on X-ray diffraction, cyclic voltammograms, and impedance response of the Si/Li2TiO3 composite electrodes. This research shows a promising anode material of Si/Li2TiO3 composite and provides more insights into the rational design of the anode materials for application in lithium-ion batteries in the future. Acknowledgements
Fig. 8. Nyquist plots and equivalent circuit of the Si/Li2TiO3 electrode during the initial lithiation/delithiation process at different voltages (a) lithiation (b) delithiation.
This work was supported by the Program of China (2011AA11A255), Natural Science Foundation of Tianjin, China (13JCZDJC32000) and the MOE Innovation Team (IRT13022).
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Evaluation of the electrochemical characteristics of silicon/lithium titanate composite as anode material for lithium ion batteries Jing Shi a , Yunhui Liang a , Linlin Li a , Yi Peng a , Huabin Yang a,b, * a Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China b Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 December 2014 Received in revised form 26 December 2014 Accepted 27 December 2014 Available online 30 December 2014
Silicon/lithium titanate (Si/Li2TiO3) nanocomposite is successfully prepared through the combination of a sol–gel approach with a high-temperature treatment as well as a high energy ball milling process. The structure and morphology of the composite are characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) analysis reveals Si particles are coated by the uniform disordered Li2TiO3 layer with a thickness of about 5 nm. The investigation in cycling performances demonstrates that Si/Li2TiO3 exhibits the improved cycling stability, with specific capacity of 471.0 mA h g1 after 50 cycles and the capacity retention is 31.5%, much higher than pure Si. Compared with pure Si, Si/Li2TiO3 shows better rate-capability, a reversible capacity of 315.2 mA h g1 at 0.8 A g1 is maintained. The higher ionic conductivity of Li2TiO3 is responsible for the improved rate performance. In addition, the results derived from XRD, the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicate that lithium ions could react reversibly with Si, and electrochemically less active Li2TiO3 turns into the Li–Ti–O ternary phase, which acts as a buffer matrix in the Si/Li2TiO3 composite, thus improving the reversibility of electrode. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Silicon/lithium titanate composite Lithium titanate matrix Electrochemical performance Anode Lithium ion battery
1. Introduction In the rapid development of portable electronic devices, lithiumion batteries (LIBs) are strongly demanded in consideration of their high energy density, long cycle life and fast charge/discharge rates. Graphite, currently used as lithium-ion battery anode, is increasingly approaching its theoretical capacity limit of 372 mA h g1, however, it is far from meeting the demands of future electronic equipment [1]. Therefore, a variety of anode materials with improved storage capacity have been intensively explored for lithium-ion batteries in the last decade. Among these, silicon has aroused extensive attention for its numerous appealing features: it has the highest specific capacity of 4200 mA h g1 [2] and is inexpensive, abundant, furthermore, the lithiation/delithiation potential of Si is slightly higher than that of lithium, resulting in the electrode with safe feature. However, there are two major problems of Si anode material: the low intrinsic electronic conductivity and huge volume expansion during Li insertion/extraction processes, resulting in cracking and crumbling of the electrode, which makes it retain poor cycling performance [3–5]. Tremendous efforts have been made to
* Corresponding author. Tel.: +86 22 23508405; fax: +86 22 23502604. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.electacta.2014.12.153 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
overcome these drawbacks, including: 1) using nanotechnologies to synthesize different Si nano/micro-structures, such as nanoparticles [6,7], nanotubes [8,9], thin films [10] and mesoporous structures [11,12]; 2) coating with carbon (C) to form Si/C composites [13,14]; 3) combining with inactive/active materials [15,16], such as metal alloys [17,18] and metal oxides [19,20]. Among other alternative anode materials, Ti-based compounds are particularly attractive for their excellent structural stability during cycling and higher flat electrode reaction voltage. Lithium titanate has emerged as a good candidate as anode material for lithium-ion batteries with higher and flatter Li insertion reaction voltage, preventing the formation of Li dendrites and the decomposition of electrolyte [21–26]. Considering the merits emerged above, anodes made of silicon/lithium titanate composites can combine the advantageous properties of silicon (high lithium-storage capacity) and lithium titanate (excellent structural stability) to improve the overall electrochemical performance of the anode for lithium-ion batteries. There have also been some researches on the composites consisting of silicon and lithium titanate in the previous reports. For example, Si/LiTi2O4 composite film was synthesized by a sol–gel method in combination with a following heat-treatment process using nano-Si powder as the raw material. The composite exhibited excellent cycle performance (1100 mA h g1 remained after 50 cycles) but a low initial coulombic efficiency
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(only about 74.8%). It was reported that the improvement of electrochemical performance was mainly induced by the porous structure of the Si/LiTi2O4 nanocomposite film, which could offer convenient channels and provided a buffer interspace to alleviate the volume expansion of Si during the cycle process [27]. Yingbin Lin et al. [28] synthesized composite a-Si film/Li4Ti5O12 via vacuum thermal evaporation technique, which shows better cycling performance than that of Li4Ti5O12 in the voltage range of 1.0–3.0 V. However, the vacuum thermal evaporation technique is difficult to achieve from the point of view of large-scale commercial applications. Li2TiO3 owns a cubic structure and can react with Li atoms to form zero-strain insertion materials. Moreover, Li2TiO3 has a threedimensional path for Li+-ion diffusion, inwhich Li+-ion migration can take place in (0 0 3) plane and along c direction [29]. The structural stability and its better ionic conductivity make it act as ideal buffer to the Si powder. Besides, the introduction of carbon can further improve the electrical conductivity of the Si/Li2TiO3 composite. In this paper, the synthesis of Si/Li2TiO3 composite is reported by dispersing micro-Si powder in Li2TiO3 matrix by a facile sol–gel method followed by a high-temperature solid-phase calcination process and high-energy ball-milling technique. According to our knowledge, there are few reports considering the effect of Li2TiO3 on the electrochemical performance of pure Si. In this work, the introduction of Li2TiO3 endows the Si/Li2TiO3 electrode with the excellent cycling stability and better rate-capability compared with pure Si. The improved electrochemical performance of the Si/Li2TiO3 electrode might be attributed to the buffer action of the Li2TiO3 matrix and the higher ionic conductivity of Li2TiO3.
ingredients dissolved in N-methyl pyrrolidinone (NMP) onto copper foil and then dried at 80 C under vacuum for 24 h. The working electrodes were assembled in 2032 coin cells using Celgard 2400 as the separator and lithium foil as the counter electrodes. 1 mol L1 LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume ratio) was employed as the electrolyte. The coin-type cells were assembled in an argon-filled glove box. The cyclic voltammetry (CV) measurement was performed on a Solartron 1287 electrochemical workstation between 0 and 3.0 V at a scan rate of 0.1 mV s1, and the test temperature was kept at 25 C. Electrochemical impedance spectroscopic (EIS) measurement was carried out by utilizing an AC voltage of 5 mV amplitude of in the frequency range of 10 kHz to 100 MHz using a Solartron 1250 impedance analyzer, and the test temperature was kept at 25 C. The galvanostatic charge/discharge tests were conducted on a LAND CT2001A battery test system at 25 C with the cut-off voltage of 0.02 and 3.0 V (vs. Li/Li+) at a specific current density of 80 mA g1. Especially, the rate test of pure Si and Si/Li2TiO3 composite at different current densities was in the voltage of 0.01 and 3.0 V (vs. Li/Li+). 3. Results and discussion 3.1. Structure characterization Fig. 1 shows the TG-DTA curves of the glucose and the Si/Li2TiO3 precursor. It can be clearly seen that four main endothermic peaks are observed at around 74, 157, 220 and 313 C in the DTA curve in
2. Experimental 2.1. Preparation of Si/Li2TiO3 composite For a typical synthesis of Si/Li2TiO3 composite, lithium acetate and tetrabutyl titanate with a molar ratio of 4:5 (Li:Ti) were dissolved in an ethanol solution. Then an appropriate amount of triethanolamine was added into the above solution, followed by an ethanol solution containing Si powder with a particle size of 1–20 mm and the molar ratio of Si to Ti was 2:1. Subsequently, an aqueous solution of glucose (C6H12O6H2O) with water to glucose molar ratio of 9:1, was added dropwise into the mixture under stirring. A suspension was achieved, then dried at 80 C for 10 h to form a dry gel precursor. The resulting precursor was pressed into round slice under pressure of 10 MPa and then heated at 700 C in argon atmosphere for 24 h. Finally, the obtained black powder was ball-milled at 400 rpm for 6 h in a 100 mL ceramic vial filled with ceramic balls and argon gas. The weight ratio of the ceramic balls to the material was maintained at 20:1. 2.2. Structural Characterization The phase of the as-prepared powders were characterized by X-ray diffraction (XRD) using a Rigaku D/max-2500 X-ray diffractometer equipped with Cu-Ka radiation. The morphologies and microstructures of the synthesized materials were characterized by a transmission electronic microscopy (TEM, FEI Tecnai20) and a scanning electron microscopy (SEM, Philips XL-30). The elementary composition was primarily determined by energy dispersive spectrum (EDS). The pyrolysis performance of the samples was studied by a thermal gravimetric analyzer (TG, SDTQ600). 2.3. Electrochemical characterization Electrodes containing Si, or Si/Li2TiO3 active materials, polyvinylidene fluoride (PVDF) binder and carbon black at a weight ratio of 8:1:1, were prepared by coating the slurry of the electrode
Fig. 1. TG-DTA curves of glucose (a) and the Si/Li2TiO3 precursor (b).
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Fig. 1(a), corresponding to the thermal behavior of glucose, which are resulted from departure of adsorbed water, fusion of the glucose and two periods of thermal decomposition of the glucose, respectively [30]. Therefore, it can be inferred that the new endothermic peak located at 497.6 C in Fig. 1(b) should be ascribed to the formation of Li–Ti–O ternary phase [31] while the other four major peaks below 350 C are relative to the thermal behavior of the glucose. The typical XRD pattern of Si/Li2TiO3 composite is displayed in Fig. 2. The strong and sharp diffraction peaks are indexed to Si and the major broaden ones can be ascribed to Li2TiO3, indicating the existence of well crystallized Si and low crystallinity Li2TiO3. However, no any peak can be indexed to the Li4Ti5O12 phase, which may be attributed to the mass loss of the rude materials during the preparation process as stated in the Experimental. As a result, the resultant Li2TiO3 doesn't match with the molar ratio of Li:Ti = 4:5. Besides, there is one discernable phase of SiO2 in the composite, which is attributed to the partial oxidation of Si. Fig. 3(a), (b) show SEM images of the Si/Li2TiO3 particles with different magnification. Many near-spherical particles are observed and this material has the typical morphology of materials prepared by sol–gel method [32] from the low-magnified SEM image (Fig. 3(a)). In the high-magnified SEM image (Fig. 3(b)), near-spherical particles can be seen with the size of 20–450 nm, including a small amount of large aggregated particles formed by the aggregation of tiny particles. Fig. 3(f) shows the energy dispersive spectrometer (EDS) pattern of the composite. It can be seen that the elements Cu, S and Si, Ti, and O, can be detected, which are originated from the Cu substrate, the contaminants of Cu substrate, and Si/Li2TiO3 particles, respectively. The EDS analysis reveals that the atom ratio of O/Ti in the Si/Li2TiO3 composite is about 3.7, which further confirms the existence of Li2TiO3 instead of Li4Ti5O12, and the molar ratio of Si to Li2TiO3 is about 3.3 (Table 1). The slight excess of O can be ascribed to the existence of O atom of SiO2. In order to further identify the Si and Li2TiO3 layer, the TEM and HR-TEM images were employed. Si presents the morphology of particles with average particle size of more than 100 nm and the uniform disordered Li2TiO3 layer with a thickness of about 5 nm can be observed in the Si/Li2TiO3 composite (Fig. 3(d) and (e)), which has been mentioned to act as a substrate for Si. Moreover, In Fig. 3(e), orderly lattice fringes are observed and the measured lattice spacing is 0.31 nm, which matches well with the (111) diffraction plane of Si. It is reasonable to state that the ordered structure is crystalline silicon. To analyze any changes in the microstructure or morphology of the composite during cycling, comparison of morphology of electrode materials before cycling and after cycling for 30 cycles are clearly shown in SEM images (Fig. 3(b) and (c)), which exhibit very little change in surface
Fig. 2. XRD pattern of Si/Li2TiO3 composite.
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morphology. Despite of the volume change, the Si/Li2TiO3 composite remains intact without breaking into smaller particles. The SEM images indicate that the overall particles could become more compact after cycling. As a result, it is particularly beneficial for maintaining the stability of the electrode materials. This is part of the reason why the good electrochemical performance can be expected. 3.2. Electrochemical performance Fig. 4displays the charge/discharge profiles of pure Si and Si/Li2TiO3 composite to compare the charge and discharge details of Si before and after the introduction of Li2TiO3. It can be seen that the charge and discharge platforms are a little different. In the sample of Si/Li2TiO3 composite, the lithium is inserted and extracted at the similar potential of Si, implying the Si plays a key role in providing most of the capacity. However, it is observed that most of the capacity loss comes from Si. While in contrast to the Si electrode, the Si/Li2TiO3 composite electrode reveals significantly enhanced cycle stability. The improved electrochemical performance can be revealed by comparing the cycling performances of pure Si and Si/Li2TiO3 composite. Fig. 4(c) compares the specific capacities of pure Si, Si/Li2TiO3 composite between 0.02 and 3.0 V up to 50 cycles. As for the pure Si, the capacity decays to 17.0 mA h g1 after 50 cycles, with capacity retention of 0.55%. It is obvious to observe a sharp drop in capacity in the 11th cycle, which can be attributed to the cracking and pulverization of Si electrode [33]. After introduced by Li2TiO3, its capacity retention is changed to 31.5%, and the retained capacity reaches 471.0 mA h g1 after 50 cycles, suggesting that the Si/Li2TiO3 electrode exhibits excellent cycle stability. Besides, it is obvious that the introduction of the Li2TiO3 significantly improves the capacity of pure Si. The Si/Li2TiO3 composite displays an initial charge capacity of 1199.3 mA h g1 and discharge capacity of 1497.9 mA h g1, an initial coulombic efficiency of 80.1%, which is much higher than that of pure Si. One possibility of the improved performance may be assigned to the buffer action of the Li2TiO3 matrix. Li2TiO3 as a zero-strain insertion material has no structural change during lithium insertion/ extraction and can effectively withstand drastic volume expansion of Si during the cycling. Besides, the Li–Ti–O ternary phase can also partly compensate the capacity loss through providing approximate 100 mA h g1 reversible lithium storage capacity [25,26,34]. In the subsequent cycles, the coulombic efficiency remains at about 97%. However, as shown in Fig. 4(d), the capacity is kept at nearly 300 mA h g1 after 100 cycles, this instability may be accounted to the following three reasons: 1) The inappropriate Li2TiO3 in the composite may not completely act as a buffer layer against the volume change of Si during repeated alloying and de-alloying; 2) The volume effect of Si can't be ignored due to the larger size of Si nanoparticles. Moreover, judging from Fig. 4(a) and (b), most of the capacity loss is caused by Si; 3) In this paper, the thermal decomposition of LiPF6 based electrolyte without other additives and the reactions of the electrolyte with the surface of the electrode materials limit the thermal stability of LIBs [35]. Fig. 5 exhibits the rate performances of pure Si and Si/Li2TiO3 composite at different current densities in the voltage range of 0.01–3.0 V. Compared with pure Si, the Si/Li2TiO3 composites shows better rate-capability. In particular, it retains a high capacity of 315.2 mA h g1 at 0.8 A g1, while that of Si is only 4 mA h g1, suggesting the much improved rate capability. Li-conductive Li2TiO3 and the 3D path for Li+-ion diffusion [36] are proposed as the reasons for a better rate capacity of Si/Li2TiO3 composite. In order to clearly understand the electrochemical properties of the lithiation and delithiation reaction, the CV, XRD and EIS tests were performed on the Si/Li2TiO3 electrode. Fig. 6 shows the CV curves of the Si/Li2TiO3 electrode at a scanning rate of 0.02 mV s1 in the potential range from 0 to 3.0 V (vs. Li+/Li). During the initial
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Fig. 3. SEM micrograph of the Li2TiO3 composite before cycling (magnification: 10000) (a), before cycling (magnification: 200000) (b) and after 30 cycles (c); TEM (d) HR-TEM (e) images of Si/Li2TiO3 particles; EDS phases (f) of Si/Li2TiO3 particles. Table 1 The results of element analysis in the composite. Element
Weight percent/%
Atomic percent/%
Si Ti
23.7 12.2
18.7 5.6
cycle, four cathodic peaks located at 1.586, 0.706, 0.123 and 0.01 V and three anodic peaks located at 0.41, 0.553 and 1.80 V are observed. The cathodic peak located at 0.706 V only emerging during the initial cycle should be attributed to the formation of the SEI film on the surface of the active particles and the decomposition of electrolyte [37,38]. The two cathodic peaks located at 0.123 and 0.01 V and the two corresponding anodic peaks located at 0.41 and 0.553 V should be ascribed to the
Fig. 4. Charge-discharge profiles of pure Si (a), Si/Li2TiO3 composite (b) and cycling performances of pure Si, Si/Li2TiO3 composite after 50 cycles (c). Si/Li2TiO3 composite after 100 cycles (d).
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Fig. 5. Rate performances of pure Si and Si/Li2TiO3 composite in the voltage range of 0.01–3.0 V.
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that of pure Si, the XRD patterns of the Si/Li2TiO3 electrode material discharging and charging to different voltages during the initial cycle are evaluated in Fig. 7. No obvious phase change has been observed when discharging from 1.586 V to 0.123 V, indicating that there is no obvious reaction between the active material Si and Li (Fig. 7(a)). However, when discharging deeply to 0.01 V (Fig. 7(b)), the XRD pattern is clearly different from the above mentioned, because the diffraction peaks of LiaSib as the main component are obviously observed while those of Si nearly disappear. The result implies that the reaction of Si with Li is apparent and most of them have been involved in the reaction when discharging to 0.01 V. When it comes to charging shown in Fig. 7(c), with the increase of the delithiation depth, the diffraction peak intensity of Si is increasing while that of LiaSib is decreasing. It can be noted that the potential interval from
reversible lithiation/delithiation reaction of Li with Si [38–40]. The reaction equation is described as follows. LixSi $ xLi+ + xe + Si
(1)
The appearance of the cathodic peak located at 1.586 V and the anodic peak located at 1.80 V suggests the existence of another reversible lithiation/delithiation reaction. Previous studies suggested Li–Ti–O ternary phase can experience an reversible Li+ ion insertion and extraction reaction in this potential interval [23,41– 43] and the lithiation reactions of Li1+xTi2-xO4 (0 X 1/3) with a spinel structure and TiO2 with a rutile structure are described as follows. Our results are in agreement with those of the previous studies. Li+ + e + Li[Li1/3Ti5/3]O4 $ Li2[Li1/3Ti5/3]O4 E = 1.56/1.65 V [44]
(2)
xLi+ + xe + TiO2 (anatase, s) $ LixTiO2 E = 1.73/2.10 V [45]
(3)
Therefore, the two peaks can be identified as the reversible lithiation and delithiation reaction of Li with Li2TiO3. The smaller area of the peaks implies the lower content of the Li2TiO3 phase in the composite, which induces the inconspicuous discharge plateau at ca. 1.5 V. Moreover, with the increase of CV scanning cycles before 15th cycle, the current value of redox peaks is gradually increasing but the potential value keeps consistent, indicating the increasing speed of lithiation/delithiation reaction. Consequently, the Si/Li2TiO3 composite with good structural stability is a promising anode material. To gain further insight into the phase change of the composite during alloying and de-alloying and to further understand why the cycling performance of Si/Li2TiO3 composite is better than
Fig. 6. Cycle voltammetry of Si/Li2TiO3 composite electrode at a scan rate of 0.2 mV s1 in the potential range from 0.0 to 3.0 V (vs. Li+/Li).
Fig. 7. XRD patterns of Si/Li2TiO3 composite during the initial lithiation/delithiation process at different voltages (a) lithiation (b) before cycling and after discharging to 0.01 V during the initial cycle (c) delithiation.
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0.41 to 1.8 V is the main stage in which LiaSib releases lithium ion. Finally, when charging to 3.0 V, the phase of LiaSib nearly disappears and Si reappears. In comparison with the XRD patterns of the Si/Li2TiO3 electrode before and after the first cycle, it is obvious that the main phases of Si and Li2TiO3 keep unchanged but portion of crystalline Si turns into amorphous structure after the lithiation/delithiation reactions. The structural change of Si leads to part of the irreversible capacity loss during the initial cycle [46]. Besides, the appearance and disappearance of the LiaSib binary phase in discharge and charge processes reveal the existence of some reversible reactions which can generate Li–Si binary phase. Meanwhile, as the CV results above show, the reaction between Li+ and Li2TiO3 are found, nevertheless, there is no obvious change of the Li2TiO3 intensity in XRD patterns shown, indicating that Li–Ti–O ternary phase acts as a buffer matrix in the Si/Li2TiO3 composite, which consists with the results obtained from the CV curves. Therefore, the main reversible reaction during the cycling process can be presented as follows. LixSi $ xLi+ + xe + Si
(4)
Lix + 2TiO3 $ xLi+ + xe + Li2TiO3
(5)
To further clarify the origin of the superior lithium storage capacity and cycling performance, EIS analyses were conducted for
the Si/Li2TiO3 composite at different potentials where redox peaks exist in CV curves during the initial cycle. The Nyquist plots of the Si/Li2TiO3 composite electrode at various charge–discharge states in the 1st cycle are presented in Fig. 8. The corresponding equivalent circuit is demonstrated as an insert in Fig. 8(b), as shown, the semicircle that is associated with the interface electrochemical polarization impedance (Rp) and a double layer capacitance at the interface between the active material and the electrolyte, is represented by Rp and CPE (a constant phase element), separately, and Re is the resistance of the electrical contacts, separator and electrolyte [47]. It can be seen clearly that all the plots possess a depressed semicircle related with the interface electrochemical polarization impedance (Rp) in high frequency (HF) and an oblique line associated with Warburg impedance (W0) in low frequency. The semicircle diameter in HF corresponds to the Rp values, which means charge transfer resistance. The Rp value raises as the lithiation depth (Fig. 8(a)) increases and it actually increases by 22.9 V mg when discharging potential reduces from 1.586 to 0.706 V. According to the CV results, it is easier to infer that the formation of the SEI film reduces the electrochemical reaction activity. In addition, the Rp value increases by 17.3 and 20.9 V mg when the discharge voltage range from 0.706 to 0.123 V and further to 0.01 V, respectively. The increase of the Rp should be attributed to the formation of Li–Si binary phase in the lithiation reaction of Si to LiaSib,and this potential interval of the lithiation reaction exsits in the XRD results. Nevertheless, the Rp decreases (Fig. 8(b)) with the increase of delithiation depth. The Rp reduces by 20.3 and 38 V mg after charging from 0.41 to 0.553 V and further to 1.8 V, respectively. The reduction of Rp mainly originates from the reversion of the LiaSib. Therefore, the conclusion can be drawn that the Si/Li2TiO3 composite anode material possesses excellent reversibility during cycling, which is consistent with the XRD and CV results. 4. Conclusions In summary, a facile and effective approach for the synthesis of nano-sized Si/Li2TiO3 composite was demonstrated via the processes of sol–gel, calcination and ball-milling. In contrast to pure Si, the cycle stability and rate capability of Si/Li2TiO3 composite are much better. It shows a capacity retention of 31.5% and a coulombic efficiency of 80.1%, along with the retained capacity of 471.0 mA h g1 after 50 cycles, which are higher than pure Si. The improved cyclability of the Si/Li2TiO3 electrode is mainly due to the buffer action of the Li2TiO3 matrix, which also ensures the uniform dispersion of Si. Additionally, the Li–Ti–O ternary phase can partly compensate the capacity loss through providing approximate reversible lithium storage capacity. The higher ionic conductivity of Li2TiO3 might be the main reason for the improved rate performance. Furthermore, the lithiation and delithiation behaviors at different potentials have been proposed based on X-ray diffraction, cyclic voltammograms, and impedance response of the Si/Li2TiO3 composite electrodes. This research shows a promising anode material of Si/Li2TiO3 composite and provides more insights into the rational design of the anode materials for application in lithium-ion batteries in the future. Acknowledgements
Fig. 8. Nyquist plots and equivalent circuit of the Si/Li2TiO3 electrode during the initial lithiation/delithiation process at different voltages (a) lithiation (b) delithiation.
This work was supported by the Program of China (2011AA11A255), Natural Science Foundation of Tianjin, China (13JCZDJC32000) and the MOE Innovation Team (IRT13022).
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